Oyster is designed as a 'proactive document cache', meaning that it takes URLs
and keeps a local copy up to date depending on user-specified criteria.

Data Model
==========

Oyster keeps its data in a MongoDB instance and makes use of GridFS to store the
raw document data.

In addition to the standard gridfs collections (fs.chunks, fs.files) oyster
uses the following collections:

tracked - metadata for tracked resources
    _id         : internal id
    _random     : a random integer used for sorting
    url         : url of resource
    versioning  : string indicating type of versioning to be used for this
                  resource.  currently set to md5 for all documents
    update_mins : minutes between automatic updates (defaults to 24*60)
    metadata    : dictionary of extra user-specified attributes

logs - capped log collection
    action    : log entry
    url       : url that action was related to
    error     : boolean error flag
    timestamp : UTC timestamp for log entry

status - internal state
    update_queue : size of update queue